Electrical device and a method of making the same



Nov. 15, 1938. L. B. BUTTERFIELD ET AL 2,136,609

ELECTRICAL DEVICE AND A METHOD OF MAKING THE SAME Filed April 10, 1936 IN VE N T0 RS 1. a. BUT TERFIEL 0 R. r. 0.4 RRE r 1' RI. PA mr JR.

A TTORIVE) Patented Nov. 15, 1938 UNITED STATES PATENT OFFICE ELECTRICAL DEVICE AND A METHOD OF MAKING THE SAME of New York Application April 10, 1936, Serial No. 73,716 5 Claims. (CL 175-41) This invention relates to electrical devices and a method of makingv the same and more particularly to high precision electric condensers, coils and the like and a method of making them. 5 Electrical devices such as condensers, coils, transformers and the like used in apparatus of high precision, e. g. such as are components of the resonant circuits employed in the communications arts, must be of a high degree of accuracy of predetermined value of electrical characteristics and of invariance of such values under service conditions. Such devices are frequently placed for protection against the effects of atmospheric changes in temperature and moisture, in containers of metal or other material and sealed therein with a fusible asphaltic mixture commonly called a ."potting" compound. Penetration of such compound into the device being "potted" may affect its electrical characteristics.

duce such devices of a simple and reliable construction in a simple and rapid manner, to have the desired accuracy and invariance of capacitance, inductance or whatever electrical characteristic may be of importance in the given device.

One embodiment of the invention contemplates covering or wrapping the device with some suitable protective covering and potting the protected device, the covering being preferably a wrapping of fibrous non-conductive non-hygroscopic material such as mineral wool, glass wool, baked textile tape, baked paper or the like, or being an inner can or casing of metal, mica, baked cardboard or the like.

Other objects and features of the invention will appear from the following detailed description of several devices constructed in accordance with the invention, taken in connection with the accompanying drawing in which the same reference numerals are applied to identical parts in the several figures and in which.

I Fig. 1 is a broken perspective view of a wrapped condenser ready to be potted;

Fig. 2 is a similar view of a container therefor, and

Fig. 3 is a similar view of acoil potted in a double container.

In the embodiment disclosed in Figs. 1 and 2, an electrical condenser has an intercalated stack III of metal and dielectric laminae held between two clamping plates 2i (only one shown) which are held together by four screws 22. Terminal members 28, 23 are attached to groups of alternate metal laminae.

An object of the present invention is to pro- The condenser proper, or other device, is assumed to have been baked or otherwise treated to drive out substantially all moisture; and, while still hot or otherwise ensured against entry of moisture thereinto, it is provided with a wrapping or covering 24 of some suitable substantially moistureless and preferably non-hygroscopic material. Mineral wool in a roughly felted, roughly sheet form has been found eminently satisfactory and may be wrapped on by hand. Preferably this wool should have been previously baked also or otherwise dehydrated, e. g. in a vacuum desiccator with calcium chloride present. Preferably also, after being wrapped, the wrapping may be secured in place with predried textile thread or tape 25 or with metallic wire, care being taken in the latter case to avoid short circuiting the terminals. However in some cases it will be found that the mineral wool will interfelt with itself while being wrapped, sufficiently to make a self retaining cover.

Other substances beside mineral wool may be used, e. g. glass wool, asbestos, or even vegetable fibre or animal fiber materials provided that these latter are carefully predried and that access of 2 moisture is barred during the process. The point is that the wrapping 24 must be substantially free of moisture when in place, must be electrically non-conductive, and must be sufllclently thick and dense to prevent the asphaltic potting compound from coming into contact with and entering between the laminae of the stack of the condenser or other electrically active elements of another kind of devicebeing potted. In the case of the condenser shown it is the edges of the laminae which must be protected; contact of the compound with the outer faces of the clamping plates 2i or with the screws 22 is harmless, unless the compound is corrosive. In the case of a coil, and particularly of a coil having a core compacted of granular magnetic material, it may be of importance to prevent access of the potting compound to the wound conductor and to the core if any. Thus the condenser, coil or other device being potted need not always be completely enclosed within the protective material so long as this is so disposed as to'protect the active parts.

If the wrapping material used needs more constraint than is conveniently aiforded by a thread, tape, wire or other strand, it may be provided with an outer wrapping of some suitable moistureless sheet material 28. Mica has been found satisfactory for this purpose; but also other materials might well be used such as predried paper or cardboard, predried thin wood veneer, or even if care be used to prevent short circuiting, a metal foil or thin sheet. The strands II if then used may be applied either over or under the sheet II as most convenient.

The wrapped condenser, or other device, is then preferably inserted into the container II which is here shown as a sheet metal box or can, open at its upper end. A measured quantity of molten potting compound 21 is placed in the can Just prior to the introduction of the device, being such in quantity that the device and compound will just substantially fill the can, the compound fiowing ordinarily over the top of the device generally within the can and thus sealing the device completely therein except for the terminals 23, 2!. While the can I! is shown as metal, it may also be of non-magnetic and electrically non-conductive material, e. g. of wood, plain cardboard, impregnated cardboard, molded resinous material, or any other mechanically and electrically suitable substance.

When the potting compound has set, the operation and the device are completed. The device proper is then completely enclosed and sealed, except for its terminals, within a seamless and completely enveloping sheath of the solidified potting compound, while at the same time the compound has not been permitted to come in contact with or enter between electrically active parts of the device and thus aiIect and alter predetermined and accurately adjusted electrical characteristics and properties of the device.

Another embodiment of the same concept of providing an electrical device with a seamless protective sheath enclosing it completely except for protruding terminals yet not in contact with the device, is disclosed in Fig. 3. Here a coil II or a condenser or other device, with or without a core, is placed in an inner can II which preferably completely encloses the coil on all six sides except for the terminals 32, II of the coil which are brought out through the can. The inner can, with the coil sealed therein by closing all seams as by soldering, gluing, crimping or otherwise sufliciently tightly to prevent entrance of potting compound, is then potted in an outer can 26 with sufiicient potting compound to enclose the inner can in a sheath of the compound complete except for the terminals 32. As before, the can It, and also the can it may be of any material appropriate to the case in hand.

In either embodiment, the potting may be done as described, by placing a sufdcient quantity of compound in the outer can and then inserting the wrapped device or the inner can containing the device into the compound; or the inner can or wrapped device may be placed or suspended in the outer can and the molten potting compound poured around it and allowed to set.

A suitable compound for use in potting is a mixture of parts by weight of asphalt melted down with 10 parts by weight of rosin and 5 parts by weight of rosin oil, although any other. satisfactory mixture may be used, there being many such well known in the art.

The embodiments disclosed are clearly illustrative only and may be departed from in many ways without departing from the scope and spirit of the invention as pointed out in and limited only by the appended claims.

What is claimed is:

1. The combination in an electrical condenser of an unimpregnated stack of metal and dielectric laminae with means to protect the same comprising a protective sheath of substantially moistureless mineral fibres substantially completely surrounding the stack, a cast sheath of moisture proof asphaltic material thereover substantially completely and seamlessly enclosing the sheath of mineral fibres on all sides and prevented from entering the stack by the sheath of mineral fibres, and a preformed casing thereover. having the cast sheath molded therein.

2. The combination in an electrical condenser of an unimpregnated stack of metal and dielectric laminae with means to protect the same comprising a protective sheath of fibrous substantially moistureless electrically non-conductive material substantially completely surrounding the stack, a wrapping of molstureless sheet material around the sheath, a cast sheath of moisture-proof asphaltic material thereover substantially completely and seamlessly enclosing the wrapping on all sides and prevented from entering the stack by the fibrous sheath and the wrapping, and a preformed casing thereover having the cast sheath molded therein.

3. The combination in an electrical condenser of an unimpregnated stack of metal and dielectric laminae with means to protect the same comprising a protective sheath of substantially moistureless mineral fibres substantially completely surrounding the stack, a wrappins of moistureless sheet material around the sheath, a cast sheath of moisture-proof asphaltic material thereover substantially completely and seamlessly enclosing the wrapping on all sides and prevented from entering the stack by the sheath of mineral fibres and the wrapping, and a preformed casing thereover having the cast sheath molded therein.

4. The combination in an electrical condenser of an unimpregnated stack of metal and dielectric laminae with means to protect the same comprising a protective sheath of mineral wool substantially completely surrounding the stack, a wrapping of sheet mica around the mineral wool sheath, a cast sheath of moisture-proof asphaltic material substantially completely and seamlessly enclosing the wrapping on all sides and prevented from entering the stack by the mineral wool sheath and the mica wrapping, and a preformed casing thereover having the cast sheath molded therein.

5. A method of making protected unimpregnated electrical condensers which comprises the steps of dehydrating a supply of fibrous electrically non-conductive material, wrapping the dried fibrous material about an unimpregnated stack of metal and dielectric laminae, and casting a substantially seamless sheath of moisture proof asphaltic material as a cover completely enclosing the wrapped device on all sides, the wrapping of fibrous material preventing the molten asphaltic material from entering the stack.

LOUIS B. BU'I'IERHEID.

ROBERT F. GARRE'I'I. ROBERT E. PARK, Jr. 

